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The Meridian Transit Of A Celestial Body Is When The Body Is Directly Passing Over The Upper Branch Or The Lower Branch Of The Observer’s Local Meridian

Upper Transit Is When The Body Passes Over The Longitude Of The Observer.

Lower Transit Is When The Body Passes Over The Longitude On The Other Side Of The Earth (180° From His Longitude). The Lower Branch Of His Meridian Is The Other Half Of A Meridian (Great Circle). It Is The Other Half Of The Great Circle That Passes Through His Position On The Other Side Of The Poles.

Although not as accurate as the GHA Method Some People Prefer To Use The DLO Method Of Determining LAN Because It Is Easier To Visualize The Passage Of The Body Over Their Meridian In Relation To The Body’s Passage Over The Central Meridian.

To Use The DLO Method Of Determining Time Of LAN – The Time Of Meridian Passage (Mer. Pass) Must Be Determined From The Lower Right Corner Of The Almanac. This Is The Time Of Passage Of The Apparent Sun Over The Central Meridian.

All That Is Required Is Finding Your DLO From The Central Meridian At The Time Of Meridian Passage And Converting This DLO Arc To Time.

Subtract the time from the time of Mer. Pass. If You Are East Of The Central Meridian.

Add the time to the time of Mer. Pass if you are West of the Central Meridian.

Example: On 31 January 1981, your 0920 zone time DR position is LAT 24° 16.0’ S, λ 151° 33.0’ E. Your vessel is on course 258° T at a speed of 18.5 kts. What is the zone time of local apparent noon (LAN)?

First find your DR position at the time of Mer. Pass. 1214 L 24° 27.0’S λ 150° 45.0’ E

Central Meridian 150° E

DR λ 150° 45.0’ E

1214 – 00s

- 3 - 00s

Ans. 1211-00

ApparentSun 1214

DLO = 0° 45.0’ of Arc to the East or 3 minutes of time

The Apparent Sun will be over the Central Meridian at 1214, So It Will Cross our DR λ 3 minutes before 1214

Example: On 22 February 1981 LAN occurs at 1147 ZT, Your Zone Description is +3. The Ho of the Sight is 73° 33.3’. You are facing due North at the time of the sight. What is the calculated Latitude at LAN?

Example: On 15 November 1981 LAN occurs at 1118 ZT, Your Zone Description is +5. The Ho of the Sight is 49° 46.0’. You are facing due South at the time of the sight. What is the calculated Latitude at LAN?